In recent years, EHCs (electrically heated catalysts) are attracting attention as an exhaust purification device that purifies exhaust gases discharged from engines of automobiles and the like. In EHCs, it is possible to forcibly activate a catalyst by electrical heating even under such conditions that the temperature of the exhaust gas is low and thus the catalyst cannot be easily activated, such as immediately after the engine is started, and thereby to enhance the purification efficiency of the exhaust gas.
An EHC disclosed in Patent literature 1 includes a cylindrical catalyst support having a honeycomb structure on which a catalyst such as platinum and palladium is supported, and a pair of surface electrodes that are electrically connected to the catalyst support and disposed opposite to each other on the outer surface of the catalyst support. In this EHC, the catalyst supported on the catalyst support is activated by electrically heating the catalyst support between the pair of surface electrodes. In this way, unburned HC (hydrocarbon), CO (carbon monoxide), NOx (nitrogen oxide), and the like contained in an exhaust gas that passes through the catalyst support are removed by the catalytic reaction.
Since an EHC is disposed on an exhaust path of an automobile or the like, the material for the above-described surface electrode needs to have, in addition to the electrical conductivity, heat resistance, oxidation resistance at a high temperature, corrosion resistance in an exhaust-gas atmosphere, and the like. Therefore, as mentioned in Patent literature 1, metallic material such as a Ni—Cr alloy and an MCrAlY alloy (M is at least one material selected from Fe, Co and Ni) is used. The surface electrode is formed on the catalyst support by thermal spraying. Meanwhile, as for the material for the above-described catalyst support, ceramic material such as SiC (silicon carbide) is used. As a result, when the EHC is electrically heated, a thermal stress occurs due to the difference between the linear expansion coefficient of the metallic material forming the surface electrode and that of the ceramic material forming the catalyst support.